These electric engines make it possible to limit the fuel mass necessary to effect the orbit transfer operation of the satellite. However, as these engines are of very low power, they have the drawback of lengthening the stationing or orbit transfer time by one to two orders of magnitude, compared with the use of chemical engines.
By virtue of this low power and the elongation of the transfer or stationing time, the control methods, which determine the thrust law of the engine (direction and amplitude as a function of time), used for chemical engines, are not applicable to electric engines.
Control methods for electric engines, known in the prior art, are either able to be carried out in the satellite without being optimal, or are optimal but require extensive operations to be carried out on the ground, these operations also being closely spaced in time. Moreover, in the event of these methods being used, any cessation of the control method or any cessation of the engine makes it necessary to start the computation again and to reschedule all of the control operations.
Control methods, carried out in the satellite, are based on a considerable simplification of the control method, and a fixed control structure is chosen, with most of the time a fixed type of orbit. One example is the use of a synchronous highly eccentric departure orbit and an inertial fixed control law. Similarly, other methods propose using a fixed control structure for transfers towards standard geostationary orbits (also known by the acronym GTO for Geostationary Transfer Orbit). By limiting the degrees of freedom fixed by the imposed control structure the determination of the control law is suboptimal. This is why the fixed control methods differ in construction from the optimal control law, which could have been determined in a continuous manner. They are therefore more expensive in fuel terms and are not more robust to disturbances and therefore require the regular rescheduling of control parameters.